Modular dc-dc converter (MDCC) has been proposed for high step-ratio interconnection in dc grid applications.
To further optimize the performance of MDCC, this paper
presents a trapezoidal current modulation with bidirectional
power flow ability. By giving all the sub-module (SM) capacitors
an equal duty to withstand the stack dc voltage, their voltages are
balanced without additional feedback control. Moreover, based
on soft-switching performance and circulating current analysis,
three-level and two-level operation modes featured with high
efficiency conversion and large power transmission, respectively,
are introduced. The control schemes of both modes are designed
to minimize the conduction losses. Besides, the SM capacitor
voltage ripples with different switching patterns are compared
and the option for ripple minimization is presented. A full-scale
case study is provided to introduce the design process and device
selection of the MDCC. The experimental tests based on a downscaled prototype are finally presented to validate the theoretical
analysis.